Top Catal (2012) 55:637–643 DOI 10.1007/s11244-012-9843-x
ORIGINAL PAPER
Green and Simple Method for Catalytic Hydrogenation of Diene-Based Polymers Yin Liu • Jialong Wu • Qinmin Pan • Garry L. Rempel
Published online: 6 July 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Catalytic hydrogenation of diene-based polymers is investigated in bulk form with different types of homogeneous and heterogeneous catalysts. Among these catalysts, we found that RhCl(PPh3)3, which could be promoted by its co-catalyst ligand (PPh3), was able to diffuse into the bulk polymer. It was shown that a required high conversion (95 mol %) was achieved within a few hours for the hydrogenation of acrylonitrile-butadiene rubber, styrene-butadiene rubber, and polybutadiene rubber using this catalyst. As an example, the hydrogenation of NBR in bulk form was investigated with respect to the effects of reaction temperature, pressure, and catalyst loading in an attempt to understand the hydrogenation of the bulk polymer. Keywords Bulk polymer hydrogenation Diene-based polymers Rhodium catalyst Solvent free reaction
1 Introduction Diene based polymers, e.g., natural rubber (NR), polyisoprene (PI), butadiene rubber (PB), styrene butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), are some of the most important polymers which are widely used as rubber in industry. Catalytic modification of these polymers, including hydrogenation, hydrosilylation, and hydroformylation etc., Y. Liu J. Wu Q. Pan (&) G. L. Rempel (&) Department of Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada e-mail:
[email protected] G. L. Rempel e-mail:
[email protected]
is of great importance in providing efficient synthetic routes for the production of novel polymers with desirable physical properties and functional groups that could widen their range of application [1–3]. Among them, the hydrogenation of diene based polymers has been most extensively investigated [4]. The hydrogenated polymers have been widely used as advanced rubber components due to their superior mechanical thermal stability and oxidative resistance [5, 6]. Currently, the catalytic hydrogenation of polymers is carried out commercially in polymer solutions in the presence of transition metal catalysts. The polymer has to be dissolved in a suitable organic solvent such as monochlorobenzene (MCB), acetone or tetrahydrofuran (THF). The usage of a large amount of organic solvent not only increases the cost of the process but also raises environmental concerns. Significant research has been carried out to improve this process by developing new catalysts [7–9] or performing the hydrogenation reaction in an environmental benign media such as ionic liquids [10, 11] or aqueous systems [12–15]. Compared with the aforementioned methods, performing the hydrogenation reaction in the bulk polymer allows a higher production capacity for a reactor of a given size. With the absence of solvent, the process will be more environmental friendly. However, the diffusion of the catalyst in the bulk polymer is greatly restricted. Therefore, as we know little research has been conducted on bulk polymer reactions, especially hydrogenation. Gilliom was the first to study catalytic hydrogenation of polystyrene-block-polybutadiene-block- polystyrene (PS–PB–PS) and 1,2-polybutadiene (1,2-PB) in bulk form using entrapped catalysts, RhCl(PPh3)3 and [Ir(COD) (PMePh2)2]PF6 [16]. The catalyst was introduced into polymer by dissolving both the polymer and catalyst together in an organic solvent and then the organic solvent
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was removed by evacuation. However, the hydrogenation reaction took a few days to obtain a desirable conversion. This tedious method to introduce the catalyst into the polymer is also not practical for a meaningful process. To improve the catalyst diffusion in a solid polymer matrix, Li et al. [17] studied the effect of supercritical CO2 on the bulk hydrogenation of NBR entrapped with the Wilkinson’s catalyst under various reaction times, reaction temperatures, hydrogen pressures, and loadings of the catalyst and the thicknesses of the polymer films. It was found that CO2 could help in improving the transport behavior of catalyst in the polymer matrices and a relatively fast reaction rate was observed. Here, we initially investigated the hydrogenation of diene based polymers in bulk form by simply mixing different solid catalysts with the polymers. Different types of catalysts and polymers were compared and it was found that the Wilkinson’s catalyst was the most effective catalyst to achieve hydrogenation in a bulk polymer system. Compared with previous methods, a high degree of hydrogenation of the polymers was achieved in a short time. Most notably, no organic solvent is required for the reaction, and many pre/post-treatment procedures are no longer necessary which makes this process very promising and robust, but simple. 2 Experimental Section NBR, containing 62 wt% butadiene (5 wt% 1,2 addition, Mw = 200,000 g/mol) and SBR, containing 75 wt% butadiene (50 wt% 1,2, addition, Mw = 300,000 g/mol) were supplied by Lanxess Inc. Polybutadiene purchased from Scientific Polymer Product (98 wt% 1,2 addition, Mw = 200,000 g/mol) was used as received. Hydrogen gas (99.999 %) was supplied by Praxair (Kitchener, Canada). RhCl(PPh3)3, OsHCl(CO)(PCy3)2O2 and RuHCl(CO) (PPh3)3 were synthesized according to the procedures described by Osborn et al., Esteruelas et al. and Vaska et al. [18–20], respectively. Rhodium on carbon (Rh/C) (5 wt%) and palladium acetate (98 %) were purchased from Sigma-Aldrich. PdCl2(PPh3)2 (99.9 %Pd) and Pd(PPh3)4 (99.9?% Pd) were purchased from Strem Chemicals, Inc. Triphenylphosphine (PPh)3 with a purity of 99 % was also purchased from Strem Chemicals Inc. The hydrogenation was carried out in an autoclave system (Parr Instrument Co., IL, USA), comprised of a 300 mL high pressure vessel with a temperature controller (±1 °C). Solid polymer was cut into particles with scissors (the average diameter of polymer particles is about 1.5 mm if not otherwise specified), and then the polymer particles were mixed with the solid catalysts. The reactor vessel was heated with an electrical heating mantle. To avoid the
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direct contact of solid polymer particle/catalyst mixture with the bottom or wall of the steel reactor that could possibly result in overheating of the sample, the mixture was located in a 5 mL vial and the vial was suspended in the reactor. The reactor was degassed with nitrogen gas. The temperature and pressure were then adjusted to the set point for the reaction. The hydrogenation degree (HD) of NBR, SBR was analyzed by FT-IR (FTS-3000; Bio-Rad Laboratories, Inc.). The hydrogenated polymer was dissolved in methyl ethyl ketone (MEK) and samples for analysis were prepared by casting a polymer film on a sodium chloride disk. The HD was calculated as previously reported in the literature [21]. The HD of PB was determined by 1H-NMR (Bruker 300 MHz Spectrometer). For NBR, the peak at 2,236 cm-1 belongs to the –CN stretching vibration. The intense peak shown at 970 cm-1 is characteristic of the level of olefin by the proton vibration of the 1,4-trans double bonds. The absorbance peak at 920 cm-1 is due to the 1,2 vinyl terminal bonds. The peak at 723 cm-1 in the hydrogenated NBR corresponds to the saturated –[CH2]n– unit (n [ 4). The relative viscosity (g*) of the hydrogenated products was measured using an Ubbelohde capillary viscometer (International Research Glassware) by dissolving 0.250 ± 0.0015 g of polymer in 25 mL of methyl ethyl ketone (MEK). Viscosity measurements were carried out at 35 °C and reported as the elution time of the sample relative to that of pure MEK [22].
3 Results and Discussion 3.1 Solid NBR Polymer Hydrogenation Using RhCl(PPh3)3 The RhCl(PPh3)3 catalyst has been found to be a very active catalyst for conventional polymer hydrogenation. Thus the initial investigation of NBR bulk hydrogenation began with this catalyst. Apart from the previous tedious method of mixing of the catalyst and polymer with a solvent followed by evacuation of the solvent from the polymer/catalyst solution [16], the catalyst and the solid NBR particles were mechanically mixed in the investigation before carrying out the hydrogenation. It was found that under a given reaction condition, NBR polymer could be effectively hydrogenated using the RhCl(PPh3)3 catalyst. Virtually complete hydrogenation of all of the olefin (93.2 mol%) can be achieved within 10 h. FT-IR analysis is consistent with this result (Fig. 1). From the resultant HNBR, the intensity of peaks at 970 and 920 cm-1 are considerably diminished while the peak at 723 cm-1 became very evident. This transformation indicates the
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639 -1
1,2 vinyl olefins (917 cm ) C-H stretching aliphatic peaks -1
1,4 trans olefins (970 cm )
Intensity (a.u.)
-1
C≡ N stretch (2236 cm )
C-C bending
a NBR Saturated -(CH2)n-units -1
(723 cm )
b HNBR 3500
3000
2500
2000
1500
1000
500
-1
Wavem number (cm ) Fig. 1 Typical FT-IR spectra of NBR and hydrogenated NBR. a NBR (0 mol% HD), b hydrogenated NBR (93.2 mol% HD) (reaction condition as entry 1 from Table 2)
reduction of the unsaturated C=C bonds to saturated C–C bonds. From the color of the HNBR, we could clearly see the catalyst had diffused into the bulk of the polymer. Even though the NBR is well above its glass transition temperature (Tg) at the reaction temperature, the molecular motion in the polymer is not sufficient to dissolve the catalyst forming a ‘‘solution’’ at the interface. The mixing should be mainly dependent on the diffusion of the catalyst within
Fig. 2 The morphology of NBR, RhCl(PPh3)3, PPh3 and resultant HNBR during the bulk hydrogenation. a NBR particles, b RhCl(PPh3)3, c PPh3, d RhCl(PPh3)3/PPh3 dispersed on the NBR
the matrix. In polymer solution hydrogenation, it has been reported that RhCl(PPh3)3 could easily dissociate one PPh3 to form the catalytic active intermediate RhCl(PPh3)2 before entering the catalytic reaction cycle [23]. This dissociation still exists in the bulk system. The dissociated PPh3 ligand promotes the diffusion of the catalytic intermediate i.e., RhCl(PPh3)2 into the polymer. Thus, upon initiation of the reaction, the dissociated catalyst ligand itself becomes the ‘‘solvent’’. To confirm it, a few experiments were also conducted in the presence of added PPh3. Under identical reaction conditions, high conversions were achieved within a short reaction time. The color of the resultant HNBR (Fig. 2f) is more uniform and the HD of NBR from the surface of the solid particle is almost the same as that from the center of the particle which also indicates that the catalyst is well diffused into the polymer particles. In addition to improving the transportation of the catalytic active intermediate into the polymer bulk, the effect of PPh3 to stablize the catalytic active species should also be considered. It has been observed that the presence of PPh3 could prevent dimerization of the catalytic active intermediate to an inactive Rh2(PPh3)4Cl2 species in the polymer solution hydrogenation [23]. However, when the weight ratio of PPh3 to catalyst is higher than 10, it has a marginal or even negative effect on the hydrogenation rate. This is probably due to the fact that the excess PPh3 shifts the equilibrium of the RhCl(PPh3)3 dissociation which inhibits the formation of the active intermediate.
particles, e HNBR using only RhCl(PPh3)3 (Table 2, entry 1), f HNBR using RhCl(PPh3)3 and extra PPh3 (Table 2, entry 2)
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640 Table 1 Relative viscosity of HNBR product
a
Weight ratio of catalyst to NBR; bWeight ratio of PPh3 to catalyst; cRelative viscosity to MEK
Table 2 Selected catalysts for the hydrogenation of bulk NBR
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Entry
WCat./WaNBR
WPPh3/WbCat.
T (°C)
P (psig)
t (h)
Blank (NBR)
0
2.65 ± 0.02
0
2.66 ± 0.05
1
0.0050
10.0
160
500
4
99.1
3.70 ± 0.05
2
0.0050
10.0
145
500
6
97.9
3.65 ± 0.10
3
0.0050
0
160
500
7
96.5
4.20 ± 0.15
4
0.0050
0
145
500
10
93.2
4.14 ± 0.10
Catalyst
WCat/WaNBR
1
RhCl(PPh3)3
0.0050
0
145
2
RhCl(PPh3)3
0.0050
10.0
3
Rh/C (5 wt %)
0.011
0
4
OsHCl(CO)(PCy3)2O2
0.0046
0
5
OsHCl(CO)(PCy3)2O2
0.0050
10.0
160
6
RuHCl(CO)(PPh3)3
0.0083
0
160
1 g of solid NBR particles was used in each experiment
7
RuHCl(CO)(PPh3)3
0.0050
10.3
160
8
(Pd (OAc)2)3
0.017
0
a
9
PdCl2(PPh3)2
0.050
10
Pd(PPh3)4
0.050
3.2 The Analysis of HNBR An important factor, the selectivity (e.g., reduction of C=C or C:N bonds) of this hydrogenation was also monitored. It has been previously shown that secondary amines that are produced by an addition of a fully saturated nitrile to an imine intermediate inhibits crosslinking of the polymer [2]. The characteristic signals for primary amines (two bands from 3,250 to 3,400 cm-1) or secondary amines (one band from 3,310 to 3,350 cm-1) did not appear in the spectra indicating the hydrogenation was completely selective towards the C=C bond in the bulk polymer system. The changes in the polymer structure such as degradation (molecular weight reduction) and cross-linking (molecular weight growth) during the reaction are very important because they can have a great impact on the physical properties (e.g., viscosity) of the HNBR product which further affects its processability. The existence of a visible gel in the HNBR could be simply detected by dissolving it in mono-chlorobenzene (MCB). Although all of the HNBR was soluble in MCB, the dissolution requires a different period of time. The HNBR produced from RhCl(PPh3)3 with the addition of PPh3 dissolved quickly while those using rhodium catalyst alone took a much longer time, indicating the microstructure of these two HNBRs were different. Therefore, viscosity measurements were used to quantitatively assess the shifts in molecular weight. The results in Table 1 show that in the presence of PPh3 during the hydrogenation
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g*c
NBR with PPh3
Entry
Weight ratio of catalyst to NBR; bWeight ratio of PPh3 to catalyst; cAverage HD
HD (mol%)
WPPh3/WbCat
T (°C)
P (psig)
t (h)
HD (mol%)
500
10
93.2c
145
500
6
97.9
160
500
5
0
160
2,000
0.8
\20
500
1
\5
2,000
1
\30
500
1
\20
70
1,000
4
0
2.5
145
500
3
0
2.5
145
500
3
0
(Entry 1 and 2), the relative viscosity of HNBR was lower than those without PPh3 (3.70 vs. 4.20). Nevertheless, the quality of HNBR produced here is still much superior to those produced using other catalysts (e.g. Os, Ru catalyst). 3.3 Effect of Other Types of Catalyst on the Bulk NBR Polymer Hydrogenation Some ruthenium, osmium, and palladium catalysts have been found to be active for the catalytic hydrogenation of diene-based polymers [24]. Therefore, these catalysts were also examined for the bulk hydrogenation process. Experimental conditions and results are shown in Table 2. No bulk hydrogenation was observed with the rhodium on carbon (Rh/C, 5 wt%) catalyst. The resultant polymer was still soluble in MCB and possessed a similar viscosity like virgin NBR, indicating that no reaction had occurred. The possible reason could be that this heterogeneous rhodium catalyst could not diffuse into the NBR matrix. It has been described in Collman’s [25] research work that homogeneous and heterogeneous catalysts behaved inversely on catalytic reactions of both polymer- and networksupported olefins. When OsHCl(CO)(PCy3)2O2 and RuHCl(CO)(PPh3)3 were used as catalysts, the hydrogenation reaction was observed, however, severe gel formation of the polymer was found in the product which was only partially soluble in MCB. The HD of the soluble part was less than 20 mol%. It has been reported by Parent et. al. [26] that
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these catalysts tend to cause crosslinking in the solution hydrogenation of NBR. The situation becomes even worse for the hydrogenation of bulk NBR as the local catalyst concentration in the bulk polymer matrix is much higher. With addition of PPh3 to the reaction system, the diffusion of the catalyst in the bulk system was greatly improved, however, no high conversion was observed. Due to the high PPh3 local concentration and its stronger affinity, the PPh3 could easily exchange with the PCy3 ligand from the Ru and Os catalysts mentioned above thus destroying their hydrogenation activity. Palladium acetate, [Pd(OAc)2]3, works well for the hydrogenation of NBR in a solvent [27]; however, no conversion was observed for the hydrogenation of NBR in bulk form. Although the potential involvement of small metal colloids i.e., Pd(0) formed from Pd(OAc)2 reduction could not be ruled out, effective hydrogenation of polymers requires a soluble catalyst. Two other types of palladium complexes, Pd(PPh3)4 and PdCl2(PPh3)2, that have the same phosphine ligands as in RhCl(PPh3)3, were also investigated for the bulk hydrogenation. No hydrogenation was observed with either of these two catalysts showing that the Pd catalysts are not effective for bulk polymer hydrogenation. 3.4 Effects of Operational Conditions on the NBR Hydrogenation Using the RhCl(PPh3)3 Catalyst Before investigation of different parameters affecting bulk hydrogenation, the mass transfer in this process was first considered. Previous kinetic studies have shown that the hydrogenation of NBR with the RhCl(PPh3)3 catalyst in homogenous solution fits a pseudo first order reaction with respect to the concentration of C=C and a ‘‘hydride path’’ was proposed for the hydrogenation mechanism [28]. The hydrogenation of NBR with Wilkinson’s catalyst in bulk form would be expected to follow the same mechanism as that in the homogeneous hydrogenation. However, from Fig. 3a (filled triangle), the conversion versus time profile deviates from a first order reaction curve that we believe is the result of a mass transfer issue. To alleviate the mass transfer limitations, finer NBR particles were prepared by dissolution of NBR in a solvent to cast a thin film on a plate; the thickness of the film was about 0.1 mm. The film was cut into small pieces after being dried under vacuum. The average thickness is only 0.1 mm which is 1/15 of the previous ones. Meanwhile, the solid NBR particle were pre-mixed with catalyst and PPh3 at elevated temperature (100 °C) under a N2 atmosphere for half an hour before commencing the hydrogenation reaction. About 96 mol% HD was achieved within 3 h for these fine NBR particles (0.1 mm 9 1 mm 9 1 mm). The results imply that the mass transfer limitation within the finer particles is much suppressed.
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The effect of the reaction pressure, temperature, and the catalyst loading are important factors which have been commonly analyzed in NBR solution hydrogenation [29]. Here, these factors were investigated with respect to the extent of hydrogenation conversion for bulk NBR hydrogenation and the results are summarized in Fig. 3. The influence of hydrogen pressure was also examined here and the results are shown in Fig. 3b. A first- to zeroorder dependence on hydrogen as the system pressure increased was observed for the bulk polymer reaction system using RhCl(PPh3)3 as catalyst. As the hydrogen pressure was varied over the range of 100–500 psig, it was found that the hydrogenation rate increased with increasing hydrogen pressure. However, no significant effect on the catalytic performance was observed at higher hydrogen pressures ([500 psig). As shown in Fig. 3c, higher HDs were obtained under higher reaction temperatures, which is consistent with previous research. In the present bulk polymer system, mass transfer of the catalyst into polymer is likely the rate controlling step for the reaction. When the temperature increases, both the intrinsic chemical reaction and the mass transfer of the catalyst into the NBR matrix are improved. The effect of the catalyst amount on the hydrogenation was also investigated and the results are shown in Fig. 3d. A high catalyst loading resulted in a fast reaction. When the weight ratio of the catalyst to NBR was lower than 1/600, the hydrogenation degree was more sensitive to the amount of catalyst and the conversion dropped sharply when the amount of catalyst was further reduced. 3.5 Hydrogenation of Other Bulk Diene-Based Polymers Finally, a comparison of hydrogenation of bulk NBR, SBR, and PB is shown in Table 3. The 1H NMR spectrum of the hydrogenated PB product showed near-complete removal of the peaks attributed to olefinic protons. Meanwhile, the infrared spectra of the hydrogenated SBR product also indicated that substantial hydrogenation had occurred. Both of these polymers undergo faster bulk hydrogenation (more than 95 mol% HD in less than 3 h) with RhCl(PPh3)3 as catalyst than that achieved with NBR. The slower reaction observed for NBR hydrogenation is attributed to the inhibiting effect of the nitrile unsaturation on the olefin hydrogenation rate. A similar inhibition by nitrile is reported by Schrock et al. [30] who identified a strong coordination of acetonitrile as being responsible for lessening the catalytic activity of a cationic rhodium complex. In addition, some preliminary research on catalyst separation from the bulk polymer was also carried out and the results showed that most of the added rhodium catalyst could be removed from the polymer by swelling the
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Fig. 3 Bulk hydrogenation of NBR using RhCl(PPh3)3 and PPh3. a WCat/WNBR = 1/200, WCat/WPPh3 = 1/10, P = 500 psig, T = 145 °C, b WCat/ WNBR = 1/200, WCat/ WPPh3 = 1/10, T = 160 °C, t = 1 h, c WCat/WNBR = 1/200, WCat/WPPh3 = 1/10, P = 500 psig, t = 1 h, d WCat/ WPPh3 = 1/10, T = 145 °C, P = 500 psig, t = 1 h. Filled square fine particles, 0.1 mm 9 1 mm 9 1 mm, filled triangle solid NBR particles with average diameter of 1.5 mm
Table 3 Comparison of bulk hydrogenation of NBR, SBR and PB Entry
WCat./WNBR
WPPh3/WCat.
t (h)
HD (mol%)
NBR
0.0050
10.0
6
97.9
SBR
0.0061
10.0
3
98.9
PB
0.0081
10.0
2
99.5
PB
0.0081
0.0
3
97.2
1 g of NBR, SBR and PB were used in different experiment, [C=C]/ [Cat.] = 2122, P = 500 psig, T = 145 °C
polymer under supercritial CO2. Details will be reported in subsequent papers.
4 Conclusion Different types of catalysts were investigated for the hydrogenation of diene-based polymers in bulk form. According to our results, the RhCl(PPh3)3 catalyst shows the highest activity for the hydrogenation of bulk NBR and the resultant HNBR produced is of high quality. It was also found the co-catalyst ligand PPh3 plays an important role in the RhCl(PPh3)3 bulk reaction. PPh3 not only promoted the diffusion of the catalyst in the bulk polymer but also stablized the catalytic active intermediate during the reaction.
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Acknowledgments We thank the Natural Science and Engineering Research Council of Canada (NSERC) and Lanxess Inc.for financial support.
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